This invention relates to laminated sheet product reinforced with composite plastic strands carrying a layer or coating (complete, partial or segmented) of heat sealable adhesive material, preferably a thermoplastic polymer, and to a process for the preparation thereof. The strands are sandwiched between layers of outer covering material which may be plastic film; foil e.g., aluminum foil; paper e g., filled or coated paper, Kraft paper, tissue paper (various plys) or various absorbent sheet-like materials. In its preferred form, the invention provides improved reinforced absorbent toweling.
The plastic strands used to reinforce the product are comprised of a core polymer of relatively high strength e.g., polypropylene or polyester. The strands carry a layer or coating of heat sealable adhesive material, such as a polymer, which at least partially covers the strands. Preferably, the strands are prepared by co-extruding or tri-extruding the composite material in a special extrusion die. The strands may be arranged in any form of reinforcing web as has been used heretofore in this art. However, it is most preferred if they are extruded in the form of plastic netting.
The invention will be described in specific detail hereinbelow with reference to its most preferred embodiment of continuous absorbent toweling comprised of multi-ply tissue reinforced with continuous tri-extruded plastic netting. The netting is sandwiched between the paper and, by means of heat bonding and pressure sealing, the toweling is prepared in a laminated sheet form.
The continuous extrusion of plastic net started in about 1956 with the process described in the Mercer U.S. Pat. No. 2,919,467. Since that time, many patents have issued in the United States as well as in other countries describing improvements and refinements in the continous extrusion process.
The initial extrusion process developed along two basic lines: the first, in which plastic sheet is extruded and holes are formed therein to provide a net-like structure, and the second, in which individual plastic strands are extruded and oriented in an interconnecting network to provide the net-like structure. This invention specifically relates to a variation in the latter of the two.
Methods for practicing the latter technique are well known. For instance, U.S. Pat. Nos. 3,700,521; 3,767,353; 3,723,218; 4,123,491; 4,152,479 and 4,190,692 show apparatus and methods for making net by continuous extrusion of strands.
The disclosures of the above-mentioned issued patents are incorporated by reference into the present specification as are all of the patents which may be referred to hereinbelow in further description of this invention.
In all of these patents at least one set of strands is extruded through a plurality of spaced individual orifices. A second set of strands in the net structure may be extruded through a second set of spaced individual orifices or a second set of strands may be preferentially extruded periodically through a continuous annular orifice slit. In all cases the two sets of strands are extruded such that the individual strands intersect at an angle and form integral joints in the extruded plastic net. The resulting extruded flat sheet in one process, or tube of plastic net in another process, is cooled to set the plastic in the strands, as for example in a water bath, and the net is drawn away from the extrusion orifices by nip rolls or other suitable drawing means. It will be obvious that the spacing between strands and strand diameter may be varied as desired.
When a tube of net is extruded, it is usually drawn over a cylindrical mandrel which may stretch the strands and enlarge the openings in the net structure. Such stretching of the strands over the mandrel preferentially orients the plastic but in practice the net is characterized as being "unoriented".
For many applications, it is desirable to further stretch the net strands and more fully orient the plastic and this may be done, where as in the case of a tube of extruded net, the tube is heated and stretched longitudinally to further elongate and orient the strands. Stretching the tube causes it to collapse while the tube is being stretched longitudinally. If the tube has been slit and formed into a flat sheet of extruded net, the flat sheet may be heated and one set of strands may be stretched and oriented in one direction, and in a second separate step, the second set of strands may be stretched to orient the strands in a second direction. Some plastic net may be oriented at room temperature but as a practical matter the net is heated to speed up and facilitate orientation of the net.